Bacterial diseases of garden beans

Rebecca Brown

Common blight, halo blight, and bacterial brown spot are bacterial leaf spot diseases of garden beans. The symptoms expressed and the life cycles of these three bacterial diseases are all very similar. Therefore, making an accurate diagnosis can be difficult. Fortunately, all three diseases can be controlled with the same practices.

In the spring, bacteria surviving in plant debris, infected the previous year, become active. These bacteria are spread from infected plant tissue to healthy plants by windblown rain, splashing water, leaf-feeding insects, tools, and people. Upon arrival, the bacteria enter plants through natural openings and wounds. Bacteria can also enter gardens in the spring on infected seed.

Fig 2: Halo blight on leaves.Photo: H. Schwartx, CSU, bugwood.org

Fig 3:Halo blight on beans.Photo: M.A. Hansen, VAPISU, bugwood.org

Common blight caused by Xanthomonas campestris pv. phaseoli, halo blight caused by Pseudomonas syringae pv. phaseolicola, and brown spot caused by Pseudomonas syringae pv. syringae infect the leaves and pods of many different common beans. Not all beans are susceptible to these three bacterial diseases.

Most bacterial leaf infections initially appear as a small angular or circular, water-soaked (dark green) spot. As these infections develop, the spots turn brown and dry, while a yellow halo of leaf tissue encircles the brown spot. Severely infected leaves often appear yellow and shredded as the dry, brown spots disintegrate or fall from the leaf. Halo blight infected leaves are usually the most yellow. Pods infected by these three bacterial diseases develop water-soaked spots, which later turn brown. It is possible to have both water-soaked spots and brown spots on the same pod.

The best way to control these bacterial diseases is through prevention. Plant commercially grown disease-free seed. Gardeners who save their own seed should not plant seed from diseased plants. Use resistant bean varieties whenever possible. Remove and dispose of all plant debris at the end of the growing season. Diseased plants should not be composted since the pile does not reach a high enough temperature over winter to kill bacterial organisms. Practice crop rotation and avoid planting beans in the same location for more than three years in a row. To minimize the spread of disease, avoid overhead irrigation and stay out of the garden when plants are wet. Remove all infected portions of the plant or the entire plant from the garden, as soon as disease is detected.